The present invention relates to a new kind of water-based resin dispersion which can be used to form film excellent in adhesion to metal surface or the like and also excellent in alkaline resistance, and relates to a resin composition for coating comprising such water-based resin dispersion as an effective ingredient.
The aluminum-zinc alloyed layer generally contains 4 to 75 wt. % of aluminum on aluminum-zinc alloyed steel plate. The balance is zinc, or zinc and trace elements added as required such as Si, Mg and Ce-La. The properties of aluminum-zinc alloyed steel plate such as corrosion resistance, heat resistance, and heat reflectivity are superior to those of galvanized steel or the like. Accordingly, the aluminum-zinc alloyed steel plate are used in various applications such as roofing material, wall material and other building materials, sink and kitchen facilities and others.
In the aluminum zinc alloyed steel plate, since aluminum is contained in the plating layer, aluminum is easily corroded when contacting with an alkaline solution, and it turns dark (the so-called darkening phenomenon occurs). When such phenomenon occurs, the appearance is extremely spoiled. Therefore, it has been required to develop a treatment process in order to prevent the darkening phenomenon even if the aluminum-zinc alloyed steel plate contacts with the alkaline solution.
Incidentally, the art of coating and treating an aluminum-zinc alloyed steel plate with a resin composition was proposed, for example, in the Japanese Official Patent Provisional Publication No. 60-197881. But this art is intended to form a film which is excellent in lubricity and corrosion resistance and is usable as an undercoat for painting, on the surface of the aluminum-zinc alloyed steel plate, and is not designed to prevent darkening due to contact with alkaline solution.